This invention generally relates to ultrasonic imaging systems and is particularly concerned with the provision of a dynamically variable electronic delay line for real time ultrasonic imaging by which the signals associated with an array of electro-mechanical acoustic transducer elements are controllably phased to thereby enable the selective scanning and dynamic focusing of a target.
In an ultrasonic imaging system of the type wherein a plurality of transmitting and receiving electro-mechanical transducer elements are disposed in an array, the scanning angle and focal distance for the acoustic signals both transmitted and received by the transducer elements can be selected by carefully controlling the timing of the signals associated with each transducer element, to the end that the acoustic pulses transmitted from each of the transducer elements of an array all arrive at the same time at a particular target disposed at a particular scanning angle and focal distance, and such that the return or "echo" signals received by each of the transducer elements are properly phased so as to be processed i.e. coherently summed, at the same time.
More specifically, and with initial reference to FIG. 1 of the appended drawings, a typical phased-array of electro-mechanical transducer elements could comprise elements 1 thru 5, each physically spaced-apart, one from the other, by a selected linear distance. Each of the electro-mechanical acoustic transducer elements 1 thru 5 serve to convert an electronic pulse signal presented thereto into an acoustic pulse and transmit same. Similarly, the return acoustic "echo" pulses received by each of the electro-mechanical transducer elements 1 thru 5 would be reconverted into electrical pulses for processing and ultimate image display. If it is desired, for example, to focus upon a remote target disposed at some angle .theta. such as indicated by reference numeral 6, it would be necessary that each of the acoustic pulses transmitted by the plurality of transducer elements 1 thru 5 arrive at the location of the remote target 6 at the same time. In this respect, it is clear that the acoustic pulses transmitted from transducer elements 2 thru 5 would have to travel a respectively increased distance represented by reference designations L2 thru L5, respectively, as compared with the acoustic pulse transmitted from acoustic transducer element 1, for example. In that the speed of the acoustic pulses travelling within the medium between the transducer elements and the remote target 6 is known, by creating a time delay between the transmission of the acoustic pulses from each of the respective transducers, time synchronism of all transmitted pulses at the remote target 6 can be assured. For example, the additional distance L5 that the acoustic pulse from transducer element 5 would have to travel can be compensated for, as contrasted with the acoustic pulse from transducer element 1, by causing the pulse transmitted by transducer element 5 to occur at a certain time interval T5 prior to the transmission of the acoustic pulse from transducer element 1.
When receiving "echo" signals that have been reflected from the target 6, similar time-delay considerations would apply. Specifically, since the signal received by transducer element 5 would be received at a time interval T5 subsequent to the receipt of the signal by transducer element 1 due to the additional distance L5 that such signal would have to travel, it would be necessary to shift or delay the signal at transducer element 1 through a time delay T5 so as to assure coherence, i.e. coincidence or proper phasing of such signal at transducer element 1 with the signal at transducer element 5. Similar considerations apply with respect to each of the signals from transducer elements 2, 3, and 4, for example, to the end that all signals received are placed into time coincidence, one with the other, for subsequent processing in accordance with conventional techniques.
As should be apparent, by altering the relative time delays of both transmission and receipt of signals at each of the respective transducer elements 1 thru 5, in effect a change in the scanning angle .theta. can be obtained. The acoustic beam transmitted and received by the plurality of transducer elements in the array can therefore be swept or steered through any desired sector, without the necessity of mechanical movement of the transducer elements per se.
In addition to the capabilities of sweeping an ultrasonic beam, a phased array of transducer elements is also required to selectively focus the beam at any desired focal distance. For example, and with reference now to FIG. 2 of the application drawings, a phased linear array of electro-mechanical transducer elements 1 thru 5 are again depicted, the array being desired in this instance to selectively focus on any given one of the remote objects or target reflectors 8 thru 18, these remote targets being disposed at varying distances from the linear array along the normal to such array in this instance.
As shown in FIG. 2, the first signal received by the array of transducer elements 1 thru 5 is that which is reflected from target 8, target 8 being at a first distance from the line of the array. The second signal received by the array of transducer elements would be that which was reflected from remote target 10, for example, and so-on, through remote target 18, each target being at a ever-increasing focal distance from the line of the array. The signals received by each of the transducer elements 1 thru 5 from the respective remote target reflectors 8 thru 18 are illustrated by the pulses as shown, with the first-received pulse such as would be obtained from the reflection from target reflector 8, for example, being that pulse to the far right-hand side of the pulse series representation, and with the last pulse received by each transducer element 1 thru 5, such as from target reflector 18, for example, being the sixth pulse, or the pulse represented at the far left-hand side of the pulse series representation for each transducer.
It is evident from these illustrations that to focus upon signals received from a target reflector 8 at the given focal distance of same, the first pulse received by transducer element 3 would have to be delayed in time for a certain interval, and the first pulse received by transducer elements 2 and 4, for example, would have to be delayed in time a somewhat shorter time interval, to the end that the first received pulse from each of the transducer elements 1 thru 5 would be in time coincidence with one another, or properly phased. The time corrections necessary to focus or phase the signals received from the different focal distances of the various target reflectors 10 thru 18 would similarly have to be adjusted, one with respect to the other, so that the second signal received by each of the transducer elements, such as that which would be reflected from target 10, would all be in time coincidence with one another for subsequent processing, and such that the third signal received by all of the transducer elements from target reflector 12 at its given focal distance would similarly be in time coincidence or proper phase with one another for subsequent processing.
In effect, the schematic illustration of FIG. 2 depicts a situation by which so-called dynamic focusing at different distances during a series of pulses are obtained, with the first pulse received being from target reflector 8, and with the last pulse received being from target reflector 18, and with the entire string of echoes respectively received by each of the transducer elements being represented by the pulse series as shown. An interesting observation can be made in this example by noting that the data vectors D.sub.1 and D.sub.5, while being the same length as one another due to the placement of the target reflectors along the normal to the linear transducer element array, are shorter than the data vector D.sub.3 representing the pulse series received by transducer element 3. The proper amount of time delay must be introduced into each channel represented by the transducer elements 1 thru 5 so that each respective corresponding echo or pulse signal in each channel are lined-up with one another. Such proper "phasing" in a dynamic focusing mode can be obtained by either effectively shortening data vector D.sub.3 to match the length of data vectors D.sub.1 and D.sub.5, or by alternatively stretching the data vectors D.sub.1 and D.sub.5 to line up with the data vector D.sub.3. Similar considerations, of course, would apply to the data vectors associated with transducer elements 2 and 4.
The first step in lining up the received signals would be to bring the first-received signal in each data channel from the target reflector closest to the array into time coincidence with one another. As is noted from the illustration, to achieve such coincidence, for the first received signals from each transducer element, a relatively long delay would be required as between transducer elements 3 and 5, or 1 and 3, with an incrementally lesser initial delay being necessary as between transducer elements 2 and 3, or 3 and 4.
Once these first-received signals are placed into time coincidence, the amount of the initial delay provided in each of the respective channels must then be repeatedly trimmed in small increments so as to bring into proper focus the subsequently received echo signals for such channel, i.e. so as to achieve a true dynamic focus or zoom lens effect. For example, the delay necessary to bring into time coincidence the second-received signals such as from target reflector 10 would not be the identical delay that was required to bring into time coincidence the first-received signals from echo reflector 8 due to the different distance of travel of the signals. Accordingly, if a given initial delay were selected for each of the respective data channels associated with each of the respective transducer elements so as to properly focus the first-received signals, this delay must then be incrementally changed by a small amount for each of the subsequently received signals so as to dynamically focus on each of the target objects in sequential order. At the ultrasonic frequencies of interest as are typically used in ultrasonic imaging environments, and considering that it is desirable to focus to an accuracy of 1/4 of a wave length or less, a suitable delay mechanism must provide incremental delay changes as short as 100 nanoseconds, and also be capable of providing initial delays as long as twelve microseconds.
In practice, the selection of the scanning or sweeping angles, as well as the selection of the focusing distances as above-discussed are achieved through the placement of a controllable variable delay line in each of the data channels associated with each respective transducer element of an array, as is depicted in FIG. 3 of the application drawings, for example. As has been shown, the proper selection of the time delay values for each of the respective delays, and the proper switching or incrementing of such delay values for each of the received pulses, readily brings-about the control of beam angle and focal length in an imaging system. This control over the time delay interval of each time delay element is effected through well-known control means so that the pulse signals received from each of the transducer elements 1 thru 5 are each respectively in phase with one another, for subsequent processing in a processor 22, and ultimate image display in a display apparatus 24 as is known.
Numerous difficulties are encountered, however, in attempting to implement the requisite delay specifications through the use of switched analog delay lines as are typically provided in prior art systems. Such switched analog delay lines resulted in portions of the signals being inserted out of sequence, a process which leads to image artifacts in the display. Furthermore, the long lumped-constant delay lines utilized in prior-art approaches are physically bulky, are quite expensive, and serve to introduce insertion losses, phase distortion, and timing inaccuracies.
Thus, while the theoretical basis of real time ultrasonic imaging is known, practical problems in the provision of delay elements capable of the high requirements imposed have inhibited the diagnostic use of such imaging with the attendant beam steering and dynamic focusing of a fixed ultrasonic transducer array.